The University of Michigan is growing its fleet of sustainable vehicles, adding three more buses to its mix of hybrids.

A hybrid bus at the University of MichiganCourtesy photo from U-M

That means that 10 of the school's 58 buses are diesel-electric hybrids that are reported to have better mileage than their diesel-only counterparts.

The new buses will replace older models from the early 2000s.

U-M introduced its first four hybrid buses in early 2012 and added three more in September 2012. The new 40-foot buses use a 25 percent less fuel and produce 25 percent fewer emissions than regular buses, according to U-M.

Since their introduction, the first seven hybrid buses have traveled more than 389,000 miles through an estimated 2.6 million rides, according to U-M figures. The first seven buses have cost about $520,000 each.

The buses use a roof-mounted battery system to supplement their diesel engines, allowing for reduced emissions. Using a generator, the bus can store energy saved while stopping at a light or stop sign and use it when accelerating, further preserving fuel.

The hybrid buses help meet President Mary Sue Coleman's goal to reduce greenhouse gas emissions, announced as part of a campus-wide sustainability initiative in 2011. When announcing the initiative, Coleman said by 2025 she expects all university buses to be hybrids.

The university expects to add between two and four fuel-efficient buses to its fleet each year.